The mitogen activated protein kinase (MAPK) signaling pathways are involved in cellular events such as growth, differentiation and stress responses (J. Biol. Chem. (1993) 268, 14553-14556). Four parallel MAPK pathways have been identified to date: ERK1/ERK2, JNK, p38 and ERK5. These pathways are linear kinase cascades in that MAPKKK phosphorylates and activates MAPKK, and MAPKK phosphorylates and activates MAPK. To date, seven MAPKK homologs (MEK1, MEK2, MKK3, MKK4/SEK, MEK5, MKK6, and MKK7) and four MAPK families (ERK1/2, JNK, p38, and ERK5) have been identified. Activation of these pathways regulates the activity of a number of substrates through phosphorylation. These substrates include: transcription factors such as TCF, c-myc, ATF2 and the AP-1 components, fos and Jun; cell surface components EGF-R; cytosolic components including PHAS-1, p90rsk, cPLA2 and c-Raf-1; and cytoskeleton components such as tau and MAP2. MAPK signaling cascades are involved in controlling cellular processes including proliferation, differentiation, apoptosis, and stress responses.
Of the known MAPK signaling pathways, the RAF-MEK-ERK pathway mediates proliferative and anti-apoptotic signaling from growth factors and oncogenic factors such as Ras and Raf mutant phenotypes that promote tumor growth, progression, and metastasis. By virtue of its central role in mediating the transmission of growth-promoting signals from multiple growth factor receptors, the RAF-MEK-ERK pathway provides molecular targets with potentially broad therapeutic applications in, for example, cancerous and noon-cancerous hyperproliferative disorders, immunomodulation and inflammation.
MEK occupies a strategic downstream position in the RAF-MEK-ERK pathway catalyzing the phosphorylation of its MAPK substrates, ERK1 and ERK2. Anderson et al. “Requirement for integration of signals from two distinct phosphorylation pathways for activation of MAP kinase.” Nature 1990, v.343, pp. 651-653. In the ERK pathway, MAPKK corresponds with MEK (MAP kinase ERK Kinase) and the MAPK corresponds with ERK (Extracellular Regulated Kinase). No substrates for MEK have been identified other than ERK1 and ERK2. Seger et al. “Purification and characterization of mitogen-activated protein kinase activator(s) from epidermal growth factor-stimulated A431 cells.” J. Biol. Chem., 1992, v. 267, pp. 14373-14381. This tight selectivity, in addition to the unique ability to act as a dual-specificity kinase, is consistent with MEK's central role in integration of signals into the MAPK pathway. MEK also appears to associate strongly with MAP kinase prior to phosphorylating it, suggesting that phosphorylation of MAP kinase by MEK may require a prior strong interaction between the two proteins. Both this requirement and the unusual specificity of MEK are suggestive that it may have enough difference in its mechanism of action to other protein kinases that selective inhibitors of MEK, possibly operating through allosteric mechanisms rather than through the usual blockade of the ATP binding site, may be found.
Constitutive action of MAPKs has been reported in >30% of primary tumor cell lines including cell lines derived from colon, lung, breast, pancreas, ovary, and kidney. Hoshino et al. “Constitutive activation of the 41-/43-kDa mitogen-activated protein kinase signaling pathway in human tumors.” Oncogene, 1999, v. 18, pp. 813-822. Higher concentrations of active MAPK/ERK (pMAPK/pERK) have been detected in tumor tissue as compared to normal adjacent tissue. Sivaraman et al. “Hyperexpression of mitogen-activated protein kinase in human breast cancer.” J. Clin. Invest., 1997, v. 99, pp. 1478-1483.
There is a continued need to find new therapeutic agents to treat human diseases. The MAPK/ERK kinases, specifically but not limited to MEK1 and MEK2, are especially attractive targets for the discovery of new therapeutics due to their important role in cancerous hyperproliferative disorders (e.g., brain, lung, squamous cell, bladder, gastric, pancreatic, breast, head, neck, renal, kidney, ovarian, prostate, colorectal, prostate, colon, epidermoid, esophageal, testicular, gynecological or thyroid cancer; non-cancerous hyperproliferative disorders (e.g., benign hyperplasia of the skin (e.g., psoriasis), restenosis, and benign prostatic hypertrophy (BPH)); pancreatitis; kidney disease; pain; preventing blastocyte implantation; treating diseases related to vasculogenesis or angiogenesis (e.g., tumor angiogenesis, acute and chronic inflammatory disease such as rheumatoid arthritis, atherosclerosis, inflammatory bowel disease, skin diseases such as psoriasis, eczema, and scleroderma, diabetes, diabetic retinopathy, retinopathy of prematurity, age-related macular degeneration, hemangioma, glioma, melanoma, Kaposi's sarcoma and ovarian, breast, lung, pancreatic, prostate, colon and epidermoid cancer); asthma; neutrophil chemotaxis; septic shock; T-cell mediated diseases where immune suppression would be of value (e.g., the prevention of organ transplant rejection, graft versus host disease, lupus erythematosus, multiple sclerosis, and rheumatoid arthritis); conditions where neutrophil influx drives tissue destruction (e.g., reperfusion injury in myocardial infarction and stroke and inflammatory arthritis); atherosclerosis; inhibition of keratinocyte responses to growth factor cocktails; chronic obstructive pulmonary disease (COPD) and other diseases.